I don't know how many stories I've heard about how injurious aikido training was in the days of the "hell dojo". I've heard of high-ranking teachers, shihan, and O sensei himself breaking people's arms, wrists, or back, dislocating shoulders, etc. My own shihan will give you a good stretching (yudansha only) -- to the point of intense pain -- if he thinks you aren't flexible enough. Once I was left with torn tendons from a nikyo he put on me. Are all these people doing aikido wrong or carelessly? No. Sometimes training hurts.

I'm not proponing intentionally injuring someone, but I'm definitely not in favor of training that is so mild, so fearful of injury, that training never comes anywhere near approximating the reality of genuine conflict.

Is chronic injury necessary to aikido training? No, but some injury is inevitable.

"Iron sharpens iron; so a man sharpens the countenance of his friend."